Digital computer systems comprise a number of subsystems and devices which must generally work together properly in order to enable correct processing of data. Digital computer systems typically include error-checking circuitry to ensure that the component subsystems and devices are operating properly. However, it will be appreciated that, if the error checking circuitry itself is not operating properly, it will be unable to verify that the other components are operating properly, and, indeed, may indicate that correctly-operating components are not operating properly.